Products for treating surfaces

ABSTRACT

Silica products obtained from trivalent metal e.g. Al compounds and silica can be used as corrosion inhibitors or adhesion promoters for organic coatings on surfaces by way of no-rinse or phosphating treatment or as pigments.

This is a continuation of application Ser. No. 07/135,437 filed Dec. 21,1987, U.S. Pat. No. 4,881,975.

BACKGROUND OF THE INVENTION

This invention relates to silica products and their use in the treatmentof surfaces prior to organic coating.

FIELD OF THE INVENTION

The treatment of surfaces, especially metal surfaces, prior to theapplication of organic coatings such as paints, lacquers, adhesives,plastics and the like, is known to be of value not only in increasingthe properties of the treated surfaces, in terms of its corrosionresistance, but also as improving the adhesion of the organic coatingsenabling them to be more effective. U.S. Pat. No. 3,506,499 describes aprocess in which an aqueous solution of chromic acid and colloidalsilica is applied to the surface formed from zinc or aluminium. BritishPatent 1,234,181 describes a process in which an aqueous solutioncomprising hexavalent chromium, trivalent chromium and silica is driedon a metal surface. The application of solutions comprising hexavalentchromium which is toxic is consequently undesirable.

SUMMARY OF THE INVENTION

We have now discovered that-mixtures of certain trivalent metals withsilica are beneficial as corrosion inhibitors or adhesion promoters forsurfaces to be finished with organic coatings.

There is provided an aqueous dispersion comprising an acidic trivalentmetal compound, in which the trivalent metal is aluminium, chromium oriron, and silica of fine particle size with a silicon to trivalent metalatom ratio of 0.2-30:1 e.g. 1.5-30:1, or a reaction product of saidcompound and silica. Preferably the dispersion also contains nickeland/or fluoride ions. There is also provided a two pack compositionwhich comprises in the first pack an aqueous dispersion of the silica,and in a second pack an aqueous solution comprising at least oneingredient which is the acidic trivalent metal compound, nickel orfluoride ion, and when the composition contains 2 or more of theingredients then at least one but not all the said ingredients can be inthe first pack, mixing at least part and preferably all of the contentsof the first and second packs producing the above aqueous dispersion oftrivalent metal compound and silica.

The present invention provides a process for coating a surface whichcomprises applying thereto said dispersion of trivalent metal and silicaand drying the dispersion on the surface to leave a coated surface; thisis a no-rinse process. The dried surface is preferably subsequentlycoated with an organic coating which is itself then dried e.g. cured bystoving at elevated temperature.

For this no-rinse process and the dispersions for use therein,significant amounts of, and especially any amount of metals in valenciesof at least 5 or metals capable of valencies of at least 5 arepreferably absent. The process and dispersions are preferablysubstantially free of oxymetal anions in which the metal has a valencyof at least 5 and are especially substantially free of said metals. Thusthe metals having 5 or 6 valency, such as molybdenum, niobium, tantalum,vanadium and tungsten e.g. in the form of their oxyanions, and chromiumin the form of hexavalent chromium are also usually absent. Mostpreferred is ferric iron or in particular aluminium, both usually in theabsence of chromium. Significant amounts of, and especially any amountof divalent metals e.g. manganese and/or alkaline earth metals such asmagnesium, are also preferably absent in the dispersion of thisinvention which are usually substantially free of alkaline earth metals.The dispersions are usually non oxidizing.

The present invention also provides a process for the treatment ofsurfaces e.g. metallic surfaces which comprises forming an adherentlayer thereon comprising at least one trivalent metal species, in whichthe trivalent metal is aluminium, ferric iron or chromium or a mixturethereof, and silica of fine particle size, the atom ratio of silicon totrivalent metal being 0.2-30:1 or a reaction product thereof, and thensubsequently rinsing the surface. Preferably the trivalent metal andsilica are in an acidic solution used to provide a conversion coating onsaid metal surface, particularly in an acidic solution used in aphosphating process with a divalent metal compound of zinc manganese orferrous iron and phosphoric acid. Alternatively the trivalent metalcation and silica may be present as such or preferably in the form of areaction product thereof as components of a paint film applied to thesurface. Accordingly there is also provided a coating composition usefulas a paint for surfaces e.g. metallic ones which comprises a paintvehicle and a preformed material formed from a trivalent metal compound,in which the metal is aluminium, ferric iron or chromium or a mixturethereof, and silica of fine particle size, or a precursor thereof, or asilicate, the silicon to trivalent metal atom ratio being 0.2-30:1, e.g.0.5-30:1, said preformed material being preferably otherwise free ofsignificant amounts of, and especially substantially free of metalscapable of having a valency of at least 5, such as those describedabove.

The trivalent metal ion is associated with an anion which does notdetract from the proposed use of the metal silica product. Preferablythe anion is such that the metal salt is water soluble e.g. sufficientlywater soluble for the proposed use. Thus in no-rinse coating dispersionsand conversion dispersions, higher water solubility is generallyrequired than is needed for the production of the paint component.Examples of suitable anions are mono and diacid phosphate, acid fluorideand acid silicofluoride. Anions such as chloride, sulphate or alkanecarboxylate are preferably not used for the no-rinse coating andconversion but are acceptable for paint use. The effect of the anion mayvary with the nature of the surface being treated and thus some anionsmay be acceptable for use on some particular surfaces but not others.The trivalent metal compound is preferably mixed with the silica (orprecursor thereof) under acid conditions. The anion of the acid isusually subject to the same criteria of solubility and non-interferencein the use of the metal silica product as is the anion with thetrivalent metal cation. Preferably the metal compound is an acid salte.g. dihydrogen phosphate, with any pH adjustment needed being byaddition of phosphoric acid or hydrogen fluoride. Most preferred areferric trisdihydrogen phosphate and aluminium tris(dihydrogenphosphate)and mixtures with phosphoric acid or the corresonding metal monohydrogen phosphate.

The silica utilised in this invention should be finely divided with afine particle size in order to ensure that it can be coated evenly overthe surface to be treated. Silica, which has been obtained pyrogenicallyfrom silicon tetrachloride, or silica, which has been precipitated in anaqueous medium from alkali silicates, may be used. The forms of silicawhich are useful in the processes of this invention may also be dictatedby the nature of the process. However crystalline forms of silica arenot useful in the processes of the invention whereas the amorphous formsof silica may be useful. Thus the silica is usually hydrophilicnon-crystalline and colloidal. It usually has Si-OH groups on thesurface and preferably only such groups, though some of these may bereplaced by Si-O-Na or Si-O-Al groups. The average surface area of thesilica is usually 10-1000 m² /g such as 30-1000 m² /g, e.g. 100-500 andespecially 150-350 m2/g. The silica particles usually have an averageparticle size less than 170 nm or but may be of 1-200 nm e.g. 1-50 nmsuch as 4-30 mm or especially 8-20 nm. Thus silica gel and precipitatedsilica may be used, but colloidal silica and fumed silica are preferredespecially for no-rinse coating and phosphating processes. In certainprocesses it may be possible or even preferable to employ a precursor ofamorphous silica i.e. a hydrated form of silica in the processes and thecompositions of this invention. The most preferred sources of silica foruse in this invention are the products which are sold under the TradeMark AEROSIL in particular those which are fumed silicas with averageparticle size of 5-20 nm and surface areas of 150-350 m² /g especially150-250 m² /g. Less preferred grades of silica for no-rinse coating andphosphating purposes are those liquid dispersion products prepared byion-exchange processes from a silicate solution, such as those soldunder the Trade Marks SYTON and LUDOX which have particle sizes of 5-30nm and 100-550 m² /g surface areas and SiOH and SiONa groups on thesurface.

The molar ratio of silica to trivalent metal (i.e. silicon to trivalentmetal) is 0.2-30:1 preferably 0.5-20:1 e.g. 1.5-20:1 such as 1.5-10.1 or3.5-30:1. For use in no-rinse coating and phosphating processespreferred molar ratios are 2.5-10:1 or 2.5-9.5:1 and especially 3-8:1 or5-8:1 or alternatively 5.5-30:1, e.g. 5.5-9.5:1, while for use inpigments in paints preferred molar ratios for silicon in silica orsilicate to trivalent metal are 1-5:1 e.g.1-10:1 such as 1-5:1 andespecially 1.5-3.5:1, though ratios of 10-30:1 can also give benefits.

In the preferred mixtures and products of the invention there is ananion which is preferably an acid phosphate; the atom ratio of trivalentmetal to P for no-rinse coating use is usually 0.2-0.45:1 e.g.0.23-0.4:1 such as 0.3-0.36:1 or 0.33-0.36:1 especially for aluminiumcompounds and 0.23-0.33:1 e.g. 0.23-0.3:1 especially for ferriccompounds. For use in the phosphate conversion process the atom ratio ofP to trivalent metal is usually 4-10:1 e.g. 4-8:1 for dispersions usedin immersion processes and 10-30:1 e.g. 15-25:1 where the solution is tobe sprayed onto the surface to be treated. For use in paints the molarratio of trivalent metal to phosphoric acid (free or combined) ispreferably 1:1-10.

The trivalent metal and silica are reacted together and/or with theunderlying substrate surface and/or with phosphoric acid or hydrofluoricacid at some stage prior to the end of the final step in the productionof the treated surface. This reaction can occur at a temperature of atleast 50° C. e.g. at least 15° C. and this temperature may be reachedeither during drying of the mixture of ingredients in a no-rinse coatingor conversion process on the surface and/or in the drying and curing ofan organic coating applied thereto subsequently. Preferably in ano-rinse coating process the dispersion is dried at elevatedtemperatures e.g. 50-300° C. and then the organic coating is cured atelevated temperature usually higher than the previous temperature e.g.at least 150° such as 150-300° C.

The reaction produced by these heating processes may be a metal coatedsilica, optionally complexed with fluoride and/or phosphate, or acomplex metal silicate or silicophosphate.

The aqueous dispersions of the invention are usually made directly fromthe trivalent metal compound and silica (or precursor thereof) Thus acompound of the metal may be dissolved in water, in the presence of acide.g. phosphoric or hydrofluoric acid if desired to give a solution ofthe required concentrations with which there is mixed the silica,preferably itself in the form of an aqueous dispersion or less preferredas finely divided solid. Products which are free from agglomeratedsilica particles are preferred for use in this invention. Where there isa tendency for the silica particles to form agglomerates we prefer toagitate the dispersion vigorously in order to reduce the size of theseagglomerates before mixing with the trivalent metal compound.Particularly for use in paints, the metal salt solution may be mixeddirectly with a precursor for the silica or alkali metal silicate inaqueous solution with subsequent adjustment of the pH if needed. Theaqueous dispersion is usually at pH 1.5-2.5 especially 1.8-2.2. Thedispersions when ready for use in no-rinse coating processes preferablycomprise of 1-20% e.g. 3-15% by weight of dissolved and dispersedmaterial, but are advantageously made in a more concentrated form with3-50% e.g. 15-40% of such material, for dilution when ready for use.Thus the dispersions may contain 0.01-0.6 g atoms/l e.g. 0.02-0.45 gatoms/l of the trivalent metal, 0.05-5 g atoms/l e.g. 0.15-3.3 g atoms/lof silica and usually 0.01-3 g atoms/l e.g. 0.06-2.0 g atoms/l ofphosphate values.

In the inorganic no-rinse coating process for pretreatment of thesurface before organic coating e.g. painting, the concentration of thetreatment solution should be sufficient to ensure that the desiredweight of coating is deposited per unit area of the surface to betreated. Preferably the dried-on film will have a weight of from 20 to5000 mg/m², more preferably from 50 to 1000 mg/m² ; coating weights foraluminium are preferably 100-300 mg/m² and for steel preferably300-700mg/m². The quantity of solution applied to the surface varieswith the method of application and the nature of the surface to be andthe concentration of the solution will be adjusted so as to provide thedesired weight of dried coating on the particular surface.

The surface to be treated may conveniently be formed from any non-porousmaterial such as metal, glass or plastic. The treatment of surfacesformed from common metal forms a preferred aspect of the presentinvention. The invention finds particular application in the treatmentof surfaces formed from iron, aluminium, tin or zinc, or alloyscomprising one or more of these metals alloyed with themselves or withother metals such as copper, nickel and/or magnesium. Examples of alloysor alloy surfaces which can be treated include stainless steel andaluminium alloys of international designations 3103 and 5052. Examplesof other particular metals include aluminium can stock, mild steel,galvanised mild steel and tin plate. The metal may be in the form of athin surface layer formed on a substrate. The substrate may be metallice.g. tin plate or non-metallic e.g. plastics material. The substrate maytake the form of individual articles, tubes, rods, wires, plates, panelsor strips. However the treatment is preferably carried out by themechanical application of a treatment solution to the metal surface andthe treatment is thereby particularly useful for the treatment of metalstrip and metal sheets. The processes may also be used to treat surfacesformed from plastics such as polypropylene, ABS plastics andpolyurethanes.

They can also be used to treat surfaces coated with plastics, paints orother organic materials to promote adhesion of a second coating layersuch as a paint or lacquer. In a preferred embodiment the processes canbe used to treat a surface layer of resin applied electrophoreticallye.g. a cataphoretic resin, which layer is subsequently to be painted. Inthis embodiment the processes find particular utility in the treatmentof films of cataphoretically applied acrylic resins, such as those soldunder the Trade Mark ELECTROCLEAR 2000, prior to subsequent painting.

The surface must be clean prior to the application of the treatmentsolutions if good results are to be obtained. Normally the surface willbe cleaned if necessary in order to remove oil, dirt and corrosionproducts and subsequently rinsed with water prior to the application ofthe solution.

Where the surface is formed of a metal, the drying of the dispersioncoating is preferably carried out by heating the metal to a temperatureof at least 50° C. such as 50-300° C. usually at least 150° C. Dryingmay be carried out at lower temperatures, such as 50-150° C. e.g.50-100° C., but, particularly when the surface is a metal surface, thisheating is especially in the presence in the dispersion of anaccelerator such as hydrofluoric acid, nickel salts, nitrates orhydrogen peroxide.

The preferred accelerator for use in the no-rinse coating processes ishydrofluoric acid. The amount of accelerator added is usually such thatthe molar ratio of silica to accelerator ion is at least 0.5:1 andpreferably at least 5:1 such as a range of 0.5:1 to 250:1. The use ofexcessive quantities of accelerator, especially hydrofluoric acid, inthe coating pretreatment processes may impart undesirable properties tothe subsequent organic coating and in general the amount of acceleratoradded will be controlled so as to avoid this. The pH of the dispersionsafter addition of the hydrofluoric acid is usually 1.3-2.3. Preferablythe amount of fluoride ion in the dispersion for coating the metalsurface is 0.5-20 g/dm³ especially 2-6 g/dm³, lower amounts necesitatinguse of high drying temperatures. Thus with 2-6 g/dm³ drying temperaturescan be 60-90° C. e.g. 70° C.

Preferably the dispersion of silica and trivalent metal compoundcontains a nickelous salt, added e.g. as nickelous sulphate instead offluoride accelerator, but preferably added as nickelous fluoride e.g. asthe tetrahydrate. Especially there is used nickelous fluoride and extrahydrofluoric acid in an atom ratio of Ni to total F of 1:2-4. Bothnickel and fluoride ion are preferably present whatever the silicon totrivalent metal atom ratio but especially when it is 0.2-5:1, e.g.1.5-3.5:1 or 1.5-20:1 such as 2.5-10:1. Amounts of nickelous ion in thedispersion to be applied to the surface are usually 0.1-20 g/l e.g.0.1-10 such as 0.1-3 g/l and especially 0.1-1 g/l or 0.1-0.6 g/l with amolar ratio of Si:Al:Ni of 90:11:1 to 1:0.5..1. When the nickel is addedas nickelous fluoride with extra fluoride the amount of nickelous andfluoride ion in the dispersion ready for application to the surfaces ispreferably 0.1-10 g/l or 0.1-3 g/l e.g. 0.1-1.0 such as 0.1-0.6 g/l themolar ratio of Si:Al:Ni:F is preferably 600:80:1:2 to 30:4..1:4. Thesebenefits are especially applicable in the treatment of steel for whichthe results can be better than with chromate pretreatment operations andalso aluminium panels e.g. for aluminium can stock.

The dispersion of the invention may be made and stored ready for use, ormay be kept in the form of a 2 pack composition in which the first packcontains the silica, and the second pack at least some of the otheringredients usually the trivalent metal compound. Any nickel and/orfluoride present can be in either pack, and if there is nickel fluoridein the second pack, the trivalent metal compound can be in either pack.However for increased storage life the first pack preferably containsthe silica and not the other ingredients, while the second packpreferably contains all the remaining ingredients. When ready to beused, at least some and preferably all the contents of the two packs aremixed, the amounts of components taken from each pack being such that onmixing the dispersions of the invention are made.

The combination of the trivalent metal and silica can also be used in aconversion coating operation. Conversion coatings involve the treatmentof a metal surface with an acidic solution which attacks the surface ofthe metal to form a strongly adhesive layer formed from metal salts.Such processes are characterised by the subsequent need to rinse excessof treatment solution to remove residual soluble compounds from thesurface of the metal in contrast to the no-rinse coating proceduresdescribed above in which the aqueous dispersion is dried onto thesurface without any subsequent removal of soluble products e.g. bywashing. A particular type of conversion coating process in which thepresent invention finds particular application is the so-calledphosphating process. Phosphating is a conversion process which is widelyused to treat surfaces formed from a variety of metals in particulariron and steel, zinc, and aluminium and alloys thereof. Previousphosphating processes involve the application of a dilute solutioncomprising one or more of the phosphates of ferrous iron, manganese orzinc in phosphoric acid to the surface of the metal in such a manner andunder conditions as to bring about the formation of a conversion coatingof phosphate salts upon the surface of the metal and subsequentlyrinsing the surface, preferably with water but optionally with chromatecontaining solutions e.g. aqueous chromic acid.

This embodiment of the processes of the present invention ischaracterised by the presence of a suitable salt of the trivalent metaland silica in the phosphating bath. The phosphating liquor can thuscontain an aqueous dispersion of the invention together with a divalentmetal compound of zinc manganese or ferrous iron and phosphoric acid.The phosphating liquor can comprise from 0.01 to 2.0 moles/litre ofsilica and from 0.0to 4.0 moles/litre of the trivalent metal, the molarratio of silica to trivalent metal being 0.2-30:1 e.g. 0.5-20:1 and themolar ratio of trivalent metal ion to divalent metal ion beingpreferably within the range 30:1 to 1:30 e.g. 3:1 to 1:3 or 2.5:1 to0.5:1 such as 1.5:1 to 1:1.5.

These phosphating liquors may comprise additives in addition to thephosphate salt of zinc, manganese or ferrous iron or mixture thereofespecially mixtures of zinc and manganese. The dispersions may alsocontain nitrites or fluorides, oxidising agents such as nitrates,chlorates or peroxides, reducing agents such as sulphites andhydroxylamine, organic additives such as quinoline toluidine andnitrophenols and heavy metals such as copper, nickel and chromium.

The phosphating is carried out by immersing the surface to be treated ina bath containing the phosphating liquor or by spraying the liquor onthe surface for a period of from 0.1 to 15 minutes e.g. 0.1-5 minutesfor spraying or 1-15 minutes more usually from 5 to 10 minutes forimmersion whilst maintaining the bath a temperature of from 40 to 110°C. e.g. 50 to 90° C. After the phosphating process is complete, thesurface is removed from the bath and rinsed with water and dried e.g. at50-300° C., such as 100-150° C. for 1-10 min.

After the coating process without rinsing or after the conversionsprocess with rinsing, there may be applied to the dried treated surfacean organic coating which is usually dried at elevated temperatures asdescribed above. The organic coating is usually a paint, lacquer, powdercoating or adhesive. Examples of suitable vehicles for the coating arethermosetting resins and preferred are alkyd resins, polyester paints,epoxy resins e.g. epoxy novolac resins, vinyl dispersions and dryingoils and may be water or organic solvent based. The drying step may besimply to remove the water or solvent but is preferably also to cure theorganic coating by stoving. Stoving paints are preferred.

Also in the organic coating applied to the surface there may be paintadditives such as pigments, fillers and chopped fibres for examplecalcium carbonate titania or glass fibre.

It has been also found that the combination of certain proportions ofsilica to trivalent metal especially aluminium, in the pretreatment ofaluminium panels gave surprising results not possessed by othercombinations of silica to the metal, and when no such change inbehaviour was obtained with comparable proportions when applied tosteel. Thus with aluminium panels e.g. of alloy 3103 atom ratios ofSi:Al in the dispersion of the invention of 3.5-20:1 and especially5.5-12:1 e.g. 6-9:1 after no rinse treatment drying, and organic coatingand curing gave much better results in adhesion tests than with atomicratios less than 3.5:1. In this case the organic coating was preferablya coil coating enamel e.g. an unsaturated polyester.

In a third aspect of the invention the trivalent metal compound andsilica may be intimately admixed and heated to a temperature of at least100° C. e.g. at least 150° C. in order to form a solid material which isuseful as a pigment in anticorrosive paints. The pigment may be made bydrying an aqueous dispersion of the silica in a solution of watersoluble salt of the trivalent metal, the dispersion being preferably atpH 1.5-6.5, using techniques such as spray drying which involve the useof elevated temperatures. Preferably however the dispersion dried is theproduct of mixing an aqueous solution of the trivalent metal salt withan alkali metal silicate in the desired molar proportions of trivalentmetal to silicon e.g. 1:1-15 and especially 1:1.5-3 and then filteringthe solid obtained, washing it thoroughly with water, and refilteringand then drying it at a temperature of at least 100° C. or at least 150°C. Instead of filtering the solid from the product suspension beforewashing, the whole suspension may be dried before the washing step.Usually in the production of the anti-corrosive material, the solutionof trivalent metal salt is added to the silicon component withadjustment of pH if needed.

The paint compositions of this invention may be formulated using anyconventional paint vehicle which maybe water or organic based and ispreferably heat curable.

Examples of useful vehicles include alkyd resins, epoxy resins anddrying oils. Preferably the paints will comprise from 15 to 50 parts byweight of the total pigment with from 1-20% by weight (of thecomposition) of the anti corrosive pigment of the invention. Materialsprepared by the admixture or reaction of a trivalent metal compound andsilica at lower temperatures may be useful as anti-corrosive pigments inpaint compositions according to this invention where the paint is astoving paint i.e. it is cured by exposure to a temperature of at least100° C., e.g. at least 150° C. An example of a material which is usefulin this respect is the material obtained by precipitating an insolublesalt of a trivalent metal from a dispersion of silica containing themetal in solution thus coating the silica particles with a layer of theprecipitated salt. The precoated or preconverted substrate carrying thedried organic coating or the substrate carrying the paint with thetrivalent metal silica material in it is often more resistant tocorrosion or loss of adhesion of the organic coating from the substratethan hitherto has had been found.

The organic coated substrate can be in the the form of steel pipes e.g.water, gas or oil pipes or formed into aluminium cans.

The invention is illustrated by the following examples:

EXAMPLE 1

Metal treatment solutions having the composition set forth below wereprepared. The trivalent metals and the phosphate were introduced as themetal dihydrogen phosphate. The aluminium dihydrogen phosphate was inthe form of a 48% w/w S.G. 1.48 solution of aluminium acid phosphate ofAl:P atom ratio of 0.34:1. The silica was pyrogenic silica sold underthe Trade Mark AEROSIL 200 with an average surface area of 200 m² /g andaverage particle size of 12 nm. It was introduced in fine aqueousdispersion. In Example G aluminium oxide was dissolved in a hydrofluoricacid solution. The solution and dispersions as described above weremixed in the required proportion to form concentrated dispersions whichwere then diluted to give dilute dispersions which were ready for use asa metal treatment solution as such or after addition of fluoride addedas 40% aqueous hydrofluoric acid.

    ______________________________________                                        EXAMPLE   M.sup.3+ g/l                                                                            PO.sub.4.sup.3- g/l                                                                     SiO.sub.2 g/l                                                                         F.sup.- g/l                             ______________________________________                                        A         Al 6.02   65        100.0                                           B         Al 3.01   33        50.0                                            C         Al 1.00   11        16.0                                            D         Al 1.00   11        16.0    8.0                                     E         Al 6.02   65        25.0                                            F         Al 3.01   33        12.5                                            G         Al 1.00   --        16.0    8.0                                     H         Fe 6.20   33        50.0                                            J         Al 1.00   11        16.0    4.0                                     K         Al 1.00   11        16.0    1.0                                     L         Al 1.00   11        16.0    0.5                                     ______________________________________                                    

The pH of the solution in Ex A-L was 1.4-2.5.

The dispersions of Examples A-L were applied to metal panels which hadbeen cleaned to give a surface free from water breaks. The panels forExample A, B, E, F, H, were of mild steel and for Ex C, D, G, J, K, L,were of aluminium. A Sheen spinner was used to give coating weightsvarying between 200 mg/m² and 800mg/m² after drying at peak metaltemperatures between 70° C. and 235° C. The panels coated with thedispersions of Ex A-C, E, F and H were dried at peak metal temperaturesof 235° C., while those of Ex. D, G, J, K, L, were dried at 70° C. Thedry panels were coated with (i) an epoxy lacquer (ii) a polyester paintor (iii) an organosol using a Sheen spinner or bar coater. The panelswere then dried at the appropriate paint stoving temperature in theregion of 200-250° C.

The painted panels were tested for adhesion by a zero T bending test andthe Erichsen test, and for corrosion by a salt spray test in accordancewith ASTM-B117. Painted mild steel panels were tested for cathodicdisbondment using 40 g/l sodium hydroxide solution at 70° C. with anapplied voltage of 5 volts for 1 hour. The results were compared tothose in which the metal phosphate/silica was replaced by the optimumcoating weights of the chromate/silica dispersions of the type describedin British Patent 1234181. The results with the aluminium panels treatedin Ex. C, D, J, K, L, were better than with chromate/silica while thosewith the rest of the Examples were comparable to use of chromate/silica.

EXAMPLE 2

250 gms of the amorphous fumed silica used in Ex 1 was dispersed in aportion of demineralised water by adding the silica in aliquots whilstvigorously stirring the dispersion. Further demineralized water to bringthe volume of water up to 1 litre was slowly added to the vessel. Theresulting dispersion was filtered through glass wool to remove the fewagglomerated large particles and leave a silica dispersion.

This dispersion was mixed into the solution of aluminium orthophosphate,used in Ex 1, in the ratio of 4:1 by volume to give a concentrateddispersion with a mole ratio of silica to aluminium of 7.46:1. Theresulting concentrate was diluted for use in the treatment processes bythe addition of five volumes of demineralised water to one volume ofconcentrate and then addition of 1% by volume of 40% aqueoushydrofluoric acid to give a aluminium silica coating or pretreatmentsolution. 15 cm ×10 cm panels of aluminium (5052 alloy) were degreasedin trichloroethylene vapour, immersed in nitric acid (10%) for 30seconds, rinsed in water, immersed in a hot, alkaline etch cleaner for10 seconds, rinsed, mechanically desmutted and rinsed with hot water.The panel was dried by spinning using a Sheen spinner. The cleanedpanels were divided into three groups (i) a control group which was notpretreated and (ii) and (iii) which were pretreated by coating byapplying a pretreatment solution on the spinner and removing the excessby spinning for 20 seconds. The panels were then dried to a peak metaltemperature of 70° C. to leave a total coating weight of 250 mg/m². Forgroup (ii) the aluminium silica pretreatment solution was used. Forgroup (iii) a chromate silica based pretreatment solution of the typedescribed in British patent 234181 was used.

One different panel from each group was coated with each of thefollowing organic finishes, each of which was supplied by EIANIndustrial Coatings

(a) a white unsaturated polyester paint (UP1230D) product

(b) an epoxy-phenolic lacquer (Konserv A133S) product

(c) an organosol (ICE 2007 111 C) product

(d) an epoxy-phenolic lacquer (IP 1602) product

The curing temperatures for finishes (a)-(d) were 160-18° C., 280° C.,270° and 200° C. respectively with curing times of 10, 1.5, 1.5 and 12minutes respectively.

All the panels were then tested for resistance to corrosion by pressinginto cans and exposure to hot acid solution. The amount of corrosion wasthen assessed by eye. In each case the group (i) untreated panels wereseverely attacked. The group (ii) panels were attacked to a lesserdegree than those of group (iii) which was much smaller than the degreeof attack found in group (i).

EXAMPLE 3

A concentrated aqueous dispersion was made up by mixing 33 parts byweight of a 30% by weight aqueous colloidal silica dispersion, 20 partsby weight of the aqueous aluminum dihydrogen phosphate solution as usedin Ex 1 and 7 parts by weight of demineralized water. The silicadispersion was sold by Monsanto as SYTON D30 and had SiONa and SiOHgroups thereon, and an average particle size of 7 nm and surface area of320 m² /g.

A pretreatment coating dispersion was made by adding 1 part by volume ofthe concentrate to 5 parts by volume demineralised water. A metalsubstrate coated with a layer of an acrylic cataphoretic resin, soldunder the Trade Mark ELECTROCLEAR 2000, was immersed in the pretreatmentsolution for 30 seconds. The substrate was removed from the pretreatmentsolution and allowed to drain and dry at ambient temperature.

A polyester/epoxyphenolic stoving paint (McPhersons No. 162 White, Stove160° C. for 10 mins) was applied to the surface of the substrate using aSheen Spinner. The coated metal was stoved at 160° C. for 10 mins. Theexperiment with the resin coated substrate was repeated but without thepretreatment coating solutions. The substrate which had been pretreatedbefore painting showed excellent paint adhesion in a cross hatch test.In comparison the substrate for which the pretreatment had been omitted,exhibited almost complete loss of paint adhesion in the cross hatchtest.

EXAMPLE 4

The process of Example 2 was repeated with the aluminium phosphatesilica concentrate being diluted with different volumes of demineralizedwater in Ex 4a, 4b, 4c, 4d, and 4e namely 1, 3, 5, 7, and 10 volumesrespectively per volume of concentrate and 1% by volume of 40% aqueoushydro fluoric acid solution being added to the diluted concentrate togive dry coating weights on the aluminium strip of 1,000, 500, 250, 180and 100 mg/m² respectively. In these experiments all the paints of Ex 2were used. Tests on the painted panels showed the results to be in thedecreasing order Ex 4d, 4e, 4c, 4b, 4a. All but Ex 4a were better thancorresponding experiments done with a chromate silica dispersion.

EXAMPLE 5

The no rinse process of Ex 4b was repeated (Ex 5a) with thin gauge mildsteel (blackplate) with the aluminium phosphate silica concentratediluted with 3 volumes of water to give a diluted dispersion to which 1%v/v of 40% hydrofluoric acid was added. The experiment was also repeated(Ex 5b) with the Aerosil 200 silica replaced by an equal weight ofAerosil OX50 fumed silica of 40 nm particle size and 50m² /g surfacearea, and the concentrate diluted with only 2 volumes of water. The drycoating weights were 500 mg/m² and 750 mg/m² for Ex 5a and 5brespectively. After drying at 70° C. the panels were coated with thefollowing finishes (i) pigmented vinyl dispersion applied to a wet filmweight of 8g/m² and cured 10 min at 210° C. in a vinyl dispersionapplied and cured as in (i) and (iii) an epoxyphenolic resin (KonservICE 200 8A with catalyst) applied to a wet film weight of 6 g/m² andcured at 280° C. for 90 sec. All the combinations of silicas andfinishes gave results better than with the corresponding unpretreatedorganic finished panels and those of Ex 5a with finish (i) gave resultsbetter than or comparable to corresponding panels with a chromate basedsilica dispersion.

EXAMPLE 6

The processes of Ex 5(a) and 5(b) were repeated (Ex 6(a) 6(b)) withcleaned panels of mild steel suitable for pipeline use and also with 2other silicas, namely (Ex 6c) Aerosil K315 which is a premade 30%aqueous dispersion of fumed silica supplied by Degussa with the silicahaving an average particle size 15 nm and surface area 170m² /g, forwhich the concentrate was diluted with 7 volumes of water beforeaddition of the hydrofluoric acid, and the layer dried at 160° C., and(Ex 6d) Aerosil MOX 170 (with the same average particle size surfacearea as K315), for which 3% v/v of the hydrofluoric acid was used. Drycoating weights were for Ex 6a-6d 500, 750, 180 and 180 mg/m2respectively.

The dried panels were lacquered with a bronze tinted clear stovinglacquer (Macphersons 4244/000) to a dry coating thickness of about 15micro metres. In cathodic disbondment tests, the pretreated paintedpanels were much better than untreated painted panel and comparable tothose with chromate silica pretreatment.

EXAMPLE 7

The process of Ex 4 (b) was repeated with mild steel panels and withpanels of aluminium alloy 3103 and varying proportions of the silicadispersion to aluminium phosphate solutions. In each case theconcentrate obtained was diluted with 3 volumes of water for the steelpanels and 5 volumes of water for the aluminium panels, beforeapplication to the metal to give coating weights of 250mg/m² foraluminium and 500 mg/m² for mild steel respectively. The organic finishwas an unsaturated polyester coil coating enamel MacPherson's 3586/105applied and cured for 10 min at 232° C. The panels were tested in a ZeroT bend test. The pretreated coated mild steels scored 7-9 out of 10 inthe test (10 being the best result) when the volume proportions ofsilica dispersions to aluminium phosphate solution varied from 9:1 to2:8 i.e. molar ratios of silica to Al of 16.6:1 to 0.46:1. Thepretreated coated aluminium panels gave the following results

    ______________________________________                                        v/v silica to aluminum                                                                      9:1     8:2     7:3   6:4  2:8                                  phosphate                                                                     molar Si:Al   16.6:1  7.4:1   4.3:1 2.8:1                                                                              0.46:1                               Score out of 10                                                                             3       7       3     0    0                                    ______________________________________                                    

EXAMPLE 8

The process of Example 4b was repeated with a mild steel panel and,instead of addition of 1% of the hydrofluoric acid there was addednickelous fluoride and hydrogen fluoride in an atom ratio Ni:F of 1:2.3to give a dispersion ready for coating the metal with an atom ratio ofSi:Al:Ni:F of 140:19:1:2.3. The dry coating weight on the metal was 500mg/m². After drying at 70° C. the panel was coated with the enamel usedin Ex 7 and stoved at 232° C. for 10 min. The pretreated painted panelwas tested for adhesion in a zero bend test and for corrosion in a 500hr. neutral salt spray test and proved to be better overall than acorresponding painted panel pretreated with the chromate silicadispersion. The pretreated painted panels were also compared in acathodic disbondment test performed as in Ex 1; the panel pretreatedwith Al, Si, Ni and F compounds gave better results than with Cr and Sicompounds.

EXAMPLE 9

In this example modifications of the process of Example 8 were used. Theaqueous silica dispersion was of 15% w/v concentration and was mixedwith the aqueous aluminium acid phosphate solution as in Ex 1 in variousproportions. The concentrates obtained were diluted with 3 volumes ofwater and the nickel fluoride and hydrofluoric acid (atom ratio Ni:F of1:2.3) as used in Ex 8. The treatment mixture obtained was applied tomild steel and dried, painted and cured as in Ex 8. In zero bend teststhe coated panels from no rinse treatment mixtures with volumeproportions of silica dispersion to aluminium phosphate solution of 9:1,8:2, 6:4, 4:6 and 2:8. Si:Al atom ratios of 10.0:1, 4.5:1, 2.6:1, 0.75:1and 0.28:1 respectively gave results comparable to those withchromate/silicate dispersions.

EXAMPLE 10

The process of Ex 4b was repeated with a 48% w/w aqueous solution offerric tris (dihydrogen phosphate) and phosphoric acid with an atomratio of Fe to P of 1:4 instead of the aluminium phosphate solution. Thecoating weights were about 500 mg/m². The volume ratios of the silicadispersion to the ferric phosphate solution were 9:1 to 1:9 giving atomratios of Si:Fe of 18.75:1 to 0.23:1. The experiments were repeated withvolume ratios of silica dispersion to ferric phosphate solution of 7:3to 1:9 giving atom ratios of Si:Fe of 4.9:1 to 0.23:1 and dilutions ofthe concentrates with 5 volumes of water to give coating weights ofabout 250mg/m². The pretreated painted panels gave good results in thezero T bend test comparable to those with chromate silica pretreatment.

EXAMPLE 11

A phosphating solution was made up by dilution to 2% v/v withdemineralized water of a concentrate obtained by reacting zinc oxide(189.5g) with phosphoric acid (SG 1.75, 472.5ml) and water (472.5 ml),and to this diluted solution was added sodium nitrite in aqueoussolution to give a concentration of nitrite of 0.02% w/v. A steel panelwas treated with this solution for 5 min at 71-82° C. rinsed with waterand dried. The process was repeated with addition to the dilutedsolution of 10% v/v of an aqueous dispersion of the Aerosil 200silica/aluminium acid phosphate concentrate of Ex 2. The resultingphosphating medium contained zinc and aluminium in an atom ratio of 1:1.

The panels were painted with Macphersons white coil coating polyesterenamel 3586/191 and cured for 10 min at 232° C. and then subjected tocorrosion tests for 500 hr in neutral salt spray. The phosphated paintedsample scored 8 out of 10 while that with extra silica and aluminiumphosphate scored 1 out of 10, the scale being from 0 (no blisteringaround diagonal scribe lines) to 10 (total covering in blisters).

EXAMPLE 12

The aluminium hydrogen phosphate solution used in Ex 1 (40 ml) wasthoroughly mixed with an aqueous 25% dispersion of Aerosil OX50 silica(as used in Ex 5) (320 ml) and aluminium oxide (28 g) and the productslurry dried at 156° C. for 5 hr. The solid was ground to a powder,washed thoroughly with water, redried and reground.

This solid was compared as an anti corrosive pigment to zinc phosphate,aluminium tripolyphosphate and a calcium silica ion exchange pigment. Ineach case 20 g of each pigment was mixed with 300 ml of acrylic stovingenamel in a ball mill and the paint obtained applied by a 24 micron barcoater to a cleaned mild steel panel. The coated panel was stoved at165° C. for 20 min, diagonally scribed and then subjected to a neutralsalt spray. The pigment from the aluminium phosphate and silica wasbetter than the others in terms of reduction in paint loss and rustspread from the scribe lines.

EXAMPLE 13

For Ex 13a aluminium hydrogen phosphate solution used in Ex 1 (40 ml)was added with stirring to an aqueous solution of sodium silicate (24 g)in deionized water (200 ml). The slurry mixture obtained was heated at180-200° C. for 2 hrs to give a dry solid which was ground to a powder,washed well with water until the filtrate had a pH greater than 5, andfinally dried for a further 2 hrs at 180-200° C.

The process was repeated with 162 g of sodium silicate and an extra 47ml of phosphoric acid (SG 1.75) (Example 13B), or 80 ml of the aluminiumphosphate solution and 24g sodium silicate in the water (Ex 13C) or 80ml of the aluminium phosphate solution and 12 g of sodium silicate withwater (Ex 13D). The process was repeated with aluminium sulphate 16hydrate (28 g) and sodium silicate (24 g) in 200 ml of deionized water(Ex13E). Finally the process of Ex 13A was repeated with 63g of 48% w/wchromic tris (dihydrogen phosphate) solution.

In each case the dried solid was ball milled at 7% w/w into MacphersonSeashell acrylic stoving enamel (3761/125) and the paint obtainedapplied to a cleaned mild steel panel with a bar coater to a thicknessof 35-40 micron. After stoving at 160° C. for 20 mins. the panels werescribed diagonally and subjected to a 500 hr salt spray test. Theresults were compared to that obtained with zinc phosphate with scoringfrom 0 (best) to 10 (worst).

    ______________________________________                                        Ex     13A    13B    13C  13D  13E  13F  Zn phosphate                         ______________________________________                                        Metal  Al     Al     Al   Al   Al   Cr   --                                   Ion                                                                           Si:M   2.24   14.9   4.6  2.24 1.13 2.24 --                                          :1     :1     :1   :1   :1   :1                                        Score  5      4      5    5    5    5    7                                    ______________________________________                                    

EXAMPLE 4

The process of Ex 2 was repeated with the replacement of the addition of1% of the hydrofluoric acid by nickelous fluoride and hydrogen fluoridein an atom ratio of Ni:F of I:2.3 to give a dispersion ready for coatingthe metal with an atom ratio of Si:Al:Ni:F of 93:12:1:2.3. The aluminiumpanels were then coated, dried painted and cured as in Ex 2. The resultswere even better than those of Ex 2 group (ii)

We claim:
 1. A process of preparing a surface for the subsequentapplication of an organic finishing coating, the process comprisingapplying an aqueous acidic dispersion to the surface and heating so asto deposit thereon an essentially inorganic priming coat of ananticorrosion or adhesion-promoting material, the dispersion comprisingan acidic trivalent metal compound in which the metal is selected fromthe group consisting of iron, aluminum and mixtures thereof, and silicaof a fine particle size, with a silicon to trivalent metal atom ratio of0.2-30:1 and substantially free of metals capable of a valency of atleast
 5. 2. The process according to claim 1, wherein the silicon totrivalent metal atom ratio is 1.5-30:1.
 3. The process according toclaim 1, wherein the coating has a weight of 20-5000 mg/m².
 4. Theprocess according to claim 2, wherein the coating has a weight of50-1000 mg/m².
 5. The process according to claim 1, wherein thedispersion is heated at 50-300° C.
 6. The process according to claim 1,wherein the surface is metallic.
 7. The process according to claim 6,wherein the surface is formed of a metal selected from the groupconsisting of iron, aluminum, tin, zinc and alloys thereof.
 8. Theprocess according to claim 6, in which the metallic surface is ofaluminum or an aluminum alloy.
 9. The process according to claim 6, inwhich the metallic surface is of steel.
 10. The process according toclaim 8, wherein the dispersion further comprise fluoride ions.
 11. Theprocess according to claim 9, wherein the dispersion further comprisesfluoride ions.
 12. A process according to claim 11, wherein thedispersion comprises 2 to 6 grams of fluoride per liter.
 13. The processaccording to claim 1 wherein the trivalent metal compound is an aluminumacid phosphate.
 14. The process according to claim 1, wherein thesurface is aluminum or an aluminum alloy; the trivalent metal compoundis aluminum acid phosphate and the silica is colloidal silica.
 15. Theprocess according to claim 1, wherein the dispersion has a pH of1.5-2.5.
 16. The process according to claim 1, wherein after saidheating which results in drying of the dispersion, the surface is coatedwith an organic coating.
 17. The process according to claim 16, whereinthe organic coating comprises a heat curable organic material which isthen cured.
 18. The process according to claim 16, wherein the trivalentmetal is selected from the group consisting of aluminum, ferric iron andmixtures thereof.
 19. The process according to claim 1, wherein thedispersion further comprises fluoride ions.
 20. The process according toclaim 19, wherein the dispersion further comprises nickel ions.
 21. Theprocess according to claim 20, wherein the dispersion comprises 0.1 to10 g/l nickel ion and 0.1 to 10 g/l fluoride ion.
 22. The processaccording to claim 1, wherein the silica has a particle size of 1 to 200nm.
 23. The process according to claim 1, wherein the silicon totrivalent metal atom ratio is 0.2-5.5.
 24. The process according toclaim 1, wherein the silicon to trivalent metal atom ratio is 1.5-5.5.